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Article Dans Une Revue Optics Express Année : 2019

Optimal birefringence distributions for imaging polarimetry

Résumé

Star test polarimetry is an imaging polarimetry technique in which an element with spatially-varying birefringence is placed in the pupil plane to encode polarization information into the point-spread function (PSF) of an imaging system. In this work, a variational calculation is performed to find the optimal birefringence distribution that effectively encodes polarization information while producing the smallest possible PSF, thus maximizing the resolution for imaging polarimetry. This optimal solution is found to be nearly equivalent to the birefringence distribution that results from a glass window being subjected to three uniformly spaced stress points at its edges, which has been used in previous star test polarimetry setups. 1. Introduction Polarimetry is the measurement of the polarization state of light and/or the polarization properties of materials. Such measurements are usually characterized in terms of the Stokes parameters and the Mueller matrix, respectively, which are directly accessible from measurements of the intensity. Imaging polarimetry, in which the polarization is measured as a function of position, is particularly important in applications ranging from microscopy to remote sensing. Conventional techniques for Stokes polarimetry require multiple intensity measurements, either through time-sequencing or by splitting different polarization components into several separate detection channels [1, 2]. For example, one common method uses a rotating quarter-wave plate (QWP) followed by a fixed linear polarizer, in which the Stokes parameters are deduced from successive intensity measurements with the QWP oriented at different angles [3]. While these techniques can produce highly accurate measurements, they can be relatively complicated and/or time-consuming, generally involving moving parts or multiple beam paths. When a short acquisition time is desirable, rapid polarization measurements may be taken using single-shot polarimetry, in which the Stokes parameters are estimated from a single intensity measurement. A variety of methods exist for single-shot polarimetry, involving gradient index lenses [4], patterned nanoscale gratings [4], or a split aperture composed of multiple polarizers [5]. One particularly simple method, referred to as star test polarimetry, uses a spatially-varying birefringent mask (BM) followed by a uniform polarization analyzer in the pupil plane of an exit-telecentric imaging system. With an appropriately chosen birefringence distribution, the inserted elements can encode full polarization information into the shape of the PSF in the rear focal plane of the lens. This method has been demonstrated experimentally using a stress-engineered optic (SEO), which is a BK7 glass window subjected to stress with trigonal symmetry at its periphery, followed by a circular analyzer [6, 7]. Natural applications for this approach are those in which the object is a sparse set of discrete points, such as in astronomy and confocal microscopy. A recent specific application was the real-time monitoring of the output polarization states of each core within a multicore fiber bundle for applications in medical endoscopes [8]. An extension of this technique is being implemented within the context of superresolution microscopy for determining the 3D position, orientation, and vibration of independent fluorescent molecules [9]. Star test polarimetry can also be applied for imaging
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Dates et versions

hal-02395513 , version 1 (05-12-2019)

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Anthony Vella, Miguel Alonso. Optimal birefringence distributions for imaging polarimetry. Optics Express, 2019, 27 (25), pp.36799. ⟨10.1364/OE.27.036799⟩. ⟨hal-02395513⟩
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